Retardation films with negative birefringence for use in liquid crystal displays (LCD) are known in prior art.
For example, in LCDs of the vertically aligned (VA) mode it is necessary to compensate light leakage that occurs in the dark state where the LC layer in the display cell is the non-selected, vertically aligned or homeotropic state. This can be achieved according to prior art by the use of a negative retardation film.
Compensated LCDs comprising a negative retardation film have been reported in prior art. For example, U.S. Pat. No. 6,141,075 discloses a VA-LCD comprising a positive and a negative retarder.
It is also known in prior art to use negative retarders comprising polymerised cholesteric LC (CLC) material that has a short helical pitch and as a result shows a Bragg reflection band in the UV range of light (UV-CLC). Such a UV-CLC layer retards visible light with negative C symmetry and thus can be used as an optical negative C retarder in the visible region of the spectrum. For example, WO 01/20393 and WO 01/20394 disclose a compensator comprising a positive and a negative retarder for use in an LCD of the VA- or the twisted nematic (TN) mode, wherein the negative retarder is for example made of polymerised UV-CLC material.
However in LC displays and LC films often the so-called “mura” effect is observed. For example, in case of a UV-CLC film, between crossed polarisers some areas of the UV-CLC films appear lighter than others, producing a speckled, non-uniform dark state. This can have a significant negative influence on the performance of UV-CLC films when used as compensator in LCDs.
The aim of the present invention is to provide a negative retarder in particular for use in LC displays that does not have the drawbacks of retardation films of prior art, shows reduced “mura” effect, is easy to manufacture and allows economic fabrication even at large scales.
Another aim is to provide a compensator comprising a negative retarder which shows good optical performance when used in LC displays, in particular improved grey level stability at wide viewing angles.
A further aim of this invention is to provide an advantageous use of a negative retarder according to this invention.
Further aims of this invention relate to liquid crystal displays, in particular to VA mode LCDs, comprising a retardation film according to the invention which show advantageous properties such as good contrast, reduced colour shift and wide viewing angles.
Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.
The above aims can be achieved by providing retarders and liquid crystal displays according to the present invention.
The inventors of the present invention have found that by using a UV-CLC film with a reduced helical pitch, in particular with a pitch of 200 nm or less, as negative retarder the mura effect can be significantly reduced. Furthermore it was found that, in an LCD comprising at least one negative UV-CLC retarder, a liquid crystal cell and two linear polarisers, the mura effect can be reduced by specific arrangment of the UV-CLC film. If the UV-CLC film is positioned such that the director orientation at its surface facing the nearest polariser is close to 45° relative to the polarisation direction, the mura effect is minimised.